This invention is a further advance pertaining to the processing of sea surface reverberation noise from the return signal of a bistatic or multistatic sonar.
Convergence zones are created by the refractive properties of the ocean. The refracted sound rays from an active transmission are reflected from the sea surface causing surface reverberation which, for a first convergence zone, can be nominally located at a range of about thirty miles from the sonar transmission source, have a two-mile width and be thought of as an extended interference source in horizontal and vertical angles made up of a multitude of spatially uncorrelated reflections.
When the surface reverberation noise signal returns along the side lobes, or the main lobe of the sonar signal, the target signal from a low doppler, submerged target located within the convergence zone is virtually indistinguishable from the surface reverberation noise signal located above it. Consequently, detection of the target by conventional sonars is virtually impossible.
Conventional beamforming with an array of hydrophones may be inadequate because the sea surface reverberation interference is sufficiently strong to limit detection even though it appears in the side lobes of the array response or because it appears close enough to the signal that it falls in the main lobe of the array response.
Heretofore, optimal (or adaptive) array processing with any of a number of optimality criteria has been suggested to reject interfering plane waves. An algorithm often implemented in practice to reject such interference has been the Least Mean Square (hereinafter also referred to as "LMS") interference canceller. A beam is steered in the direction of interest using all hydrophones but one which is used as a reference hydrophone. The output of the reference hydrophone is passed through a finite impulse response filter with adaptive coefficients and the filter output subtracted from the beam output. The finite impulse response coefficients are updated using the LMS algorithm to minimize the mean square value of the difference. That is, the beam output serves as the primary input and the reference hydrophone signal as the input to a LMS adaptive noise canceller. When the plane wave interference is much stronger than the background noise and the sonar target signal, this approach provides effective rejection of the interference.
However, in some sonar applications, the interference, though highly directional, cannot be modeled as one or more discrete plane wave fronts but is actually continuous over some finite but narrow region. Consequently, if a single reference hydrophone is used, only a single null is generated which cannot completely suppress the extended interference source, particularly, if the interference source extent is large compared to the width of the null. Thus, as additional reference hydrophones are added to the LMS canceller structure, more nulls can be directed toward the interference improving the cancellation but never completely cancelling the interference. However, as the nulls become closely spaced over the interference extent, additional reference hydrophones will not improve cancellation significantly. Thus, since each additional reference is costly in terms of computations in the adaptive algorithm, there is a tradeoff between computational cost and cancellation performance. Moreover, since algorithm noise is proportional to the number of references, a small improvement in cancellation resulting from the addition of references may actually be offset by the increased algorithm noise. Furthermore, when the target is located within the surface reverberation extent, the target signal as well as the interference will be suppressed, thus making it very difficult to detect such a target.
It is therefore an object of the present invention to provide an active sonar system adapted to detect the presence of a low doppler target located within the convergence zone of the sonar in the presence of surface reverberation noise.
Another object of the invention is to provide a bistatic sonar system adapted to extract the correlation properties in the "signal present" and "signal absent" cases for the energy received at primary and reference sensors.
A further object of the invention is to provide a bistatic sonar whose reference sensor is spatially separated from the primary sensor a substantial distance, and whose processor is adapted to exploit the different correlation properties of the surface reverberation energy and the target signal received at the primary and reference sensors.
A further object of the invention is to provide a bistatic sonar processor having simplified computational requirements.
Another object of the invention is to provide a sonar processor employing a single reference hydrophone and adaptive filter with a primary array for detecting the presence of a submerged target in the convergence zone.
Yet another object of the invention is to provide a sonar processor employing an adaptive filter canceller structure adapted for signal predictor operation.